1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a magnetic layer and a back coating layer, on a substrate and more particularly, the present invention is directed to the improvement in the traveling stability and durability of a magnetic recording tape for video, audio or computer use which is set in a cassette and, in use, wound around a rotating drum which carries a magnetic head.
2. Description of the Related Art
In general, a magnetic tape is prepared by coating a magnetic paint comprising magnetic powder, a binder resin, an organic solvent and necessary additives on one surface of a substrate film, such as a polyester film, drying the coated paint to form a magnetic layer, coating a back coating paint comprising a non-magnetic powder, a binder resin, an organic solvent and necessary additives on the other or back surface of the substrate and drying the coated back coating paint to form a back coating layer. The magnetic tape is required to have good traveling stability and durability.
Heretofore, it has been believed that the traveling stability and durability of the magnetic recording tape was controlled by curing characteristics of the binder resin in the magnetic layer and/or film strength of the magnetic layer. Accordingly, many proposals have been made on evaluation of the curing characteristics of the binder resin and the surface strength of the magnetic layer. For example, as disclosed in Japanese Patent Kokai Publication No. 182546/1988, a magnetic recording tape having a magnetic layer comprising a binder resin cured with a curing agent is dipped in a solvent, such as a halogenated hydrocarbon, an ether or a ketone, and a relationship between extracted materials and an abraded amount of a ball in a scratch test is sought.
However, with such an evaluation method of the surface strength of the magnetic layer, stiffness of the magnetic recording tape can be evaluated but the traveling stability and durability cannot be accurately evaluated since tackiness on the tape surface is not taken into consideration. That is, with such an evaluation method, since the extracted materials are treated as a whole, it is impossible to distinguish low molecular weight compounds, such as lubricants which have actually good influence on the traveling stability, from other compounds. The above evaluation method cannot measure the amount of the components which deteriorate the traveling stability and durability and therefore does not necessarily reveal the relationship between the traveling stability and/or the durability and the surface strength of the magnetic layer. In other words, in the case of the magnetic recording tape having the magnetic layer with ideal surface strength, when the extracted materials include a component having a viscosity in a specific range, the magnetic layer has a high degree of tackiness and resistance between the magnetic recording tape and parts of a recording and reproducing equipment with which the magnetic recording tape come into contact. Thus, the traveling stability and durability are not satisfactorily improved. Accordingly, from the evaluation of the surface strength of the magnetic layer by the conventional method, the traveling stability and durability of the magnetic recording tape cannot be accurately evaluated, and the structure of the magnetic recording tape having an improved traveling stability and durability may not be clarified.